Thermoelastic properties of ice VII and its high-pressure polymorphs: Implications for dynamics of cold slab subduction in the lower mantle

Acoustic velocities in polycrystalline H₂O ice have been measured at room temperature in a pressure range 6-60GPa by a Brillouin scattering method. Synchrotron X-ray diffraction measurements were also conducted simultaneously with the Brillouin scattering measurements in a pressure range 40-60GPa. The obtained elastic moduli of high-pressure ice indicate that bcc-structured ice undergoes two transitions related to a change in the hydrogen bonding state at approximately 40GPa and 58GPa, i.e. transitions of ice VII to the pre-transitional state of ice VII at 40GPa and to the dynamically disordered ice X at 58GPa, respectively. This observation is consistent with previous spectroscopic studies and X-ray diffraction studies. Pressure dependencies of adiabatic elastic moduli and specific heat for ice VII and its high-pressure polymorphs were obtained from the acoustic velocity and volume data measured in this study. Isothermal compression data were obtained from previous studies. The relationships between pressure and adiabatic elastic moduli for ice VII were obtained as follows: K_s=4.0(2)+8.51(4)×P-0.081(2)×P²+5.2(4)×10^-4×P³; μ_s=14(2)+1.7(3)×P-0.04(2)×P²+5(3)×10^-4×P³. An empirical relationship between specific heat and pressure at room temperature was obtained for ice VII as follows: C_p=3.3(2)+22.1(1)×e^-0.058(2)P. This result implies that the transition from ice VII to the dynamically disordered ice X is accompanied with a discontinuous change in several thermodynamic properties of ice. The elasticity difference between ice VII and the dynamically disordered ice X may affect the dynamics of cold subducting slabs in Earth's lower mantle and the interiors of icy planets. The thermoelastic properties of high-pressure polymorphs of ice obtained in this study could contribute to clarifying the dynamics and the evolution of Earth and icy planets and satellites.